The invention is directed to a method and apparatus for coating sheet material and particularly to a method and apparatus for coating corrugated material with a curable coating free of optical defects, and the coated article.
High strength sheet materials may be continuously corrugated by directing thermo formable sheet material between a pair of interdigitated forming rollers and then through a closely spaced cooled calibrator having the profile corresponding to the desired final shape. Such a system is described in United States Patent Application of Willie Charles Paul entitled "Method and Apparatus for Corrugating Sheet Materials", Ser. No. 07/960,489, filed Oct. 13, 1992, now U.S. Pat. No. 5,340,518 Attorney Docket No. 8CS-5377, assigned to the assignee herein, the teachings of which are incorporated herein by reference. The resulting product has good optical properties and is strong.
Clear, corrugated sheet is useful in greenhouse applications. However, it is necessary to coat the sheet material with an anti-drip, condensate coating. The coating is a wetting agent that allows moisture in the greenhouse atmosphere to condense on the underside of the sheet without fogging and without dripping. The condensed moisture then flows along the corrugated sheet to a collector or gutter structure, near a lower end. It is difficult to uniformly coat corrugated sheet materials because the shape is irregular. For the same reason, corrugated sheet is difficult to doctor. Also, the coating tends to form bubbles which reduce the transparency and optical clarity which is undesirable. Various coating techniques have proved ineffective, including spraying, spraying and thereafter applying a doctor blade or spraying and thereafter doctoring with an air knife. A need therefore exists for a method and apparatus for coating corrugated sheet materials with a curable coating free of optical defects.